A DNA molecule corresponding to a specific yeast (Saccharomyces cerevisiae) chromosome will be isolated. The general arrangement of base sequences will be determined by partial denaturation mapping and the nature of the base sequences at the molecular termini will be examined. Attempts will be made to carry out DNA heteroduplex mapping employing yeast strains carrying deletions. These experiment would provide information about the continuity of the DNA at the centromere, the absolute size of the deletions, and the relationship of recombination units and physical distance. The precision with which sites for the initiation of replication on a DNA molecule are chosen will be investigated by electron microscope. Initiation sites activated on a particular molecule throughout the S period will be determined. Variation in the length of the S period in cells growing with very different cell cycle times will be examined. An analysis of the organization of proteins in chromatin fibers will be begun. DNA-binding proteins will be isolated and analyzed for alterations in replication-defective mutants. Because of its ease of isolation and high concentration in the cell, one of these proteins, DNA unwinding protein, will be purified and studied. Its time of synthesis in the cell cycle, and rate of synthesis in mitotic and meiotic cells will be determined. Finally, specific antisera will be used to localize DNA unwinding protein in chromatin fibers.